The field of the invention relates to particles especially 3-D structural color pigments based on solid colloidal crystals of monodispersed spheres, desirably silica spheres, and the manufacturing processes suitable for large-scale production of such solid colloidal crystal products.
The 3-D structural color pigments yield color effects as a result of light diffraction by ordered three-dimensional structures of colloidal silica spheres. They diffract light because the lattice spacings, and therefore, the modulation of refractive index in their structures are in the range of the wavelength of light. The color effects are optimized by adjusting the refractive index difference between the silica spheres and the media in between the spheres. This may be accomplished by partially or completely infiltrating the silica sphere based crystal structure with one or more suitable secondary materials. The resultant high quality colloidal crystals may also be used as photonic crystals for potential photonic and optoelectronic applications.
By way of background, precious opals are well known for their striking color displays. The strong color effect by these natural gemstones typically originates from their unique structures formed by closely packed, uniformly sized silica spheres (Sanders J V, Nature 1964, 204, 1151-1153; Acta Crystallogr. 1968, 24, 427-434). These highly organized structures (super-lattices of silica spheres) with the size of the spheres in the range of wavelength of visible light selectively diffract certain wavelengths and, as a result, provide strong, angle dependent colors corresponding to the diffracted wavelengths. Synthetic opals were produced by crystallizing uniformly sized silica spheres mainly through sedimentation processes (see for example: U.S. Pat. No. 3,497,367). These synthetic gemstones are used as alternatives to the natural opals in the jewelry industries.
Recently, scientists, mainly in academic institutions have discovered that materials with opal-like structures may be used as photonic band gap materials or crystals. An ideal photonic band gap crystal has the capability to manipulate light (photons) the same way as semiconductors manipulate electrons(see John D. Joannopoulos, et al. xe2x80x9cPhotonic Crystals, Molding of the Lightxe2x80x9d, Princeton University Press, and Costas M. Soukoulis (ed.), xe2x80x9cPhotonic Band Gap Materialsxe2x80x9d, NATO ASI Series E, Vol. 315, Kluwer Academic Publishers). These crystals with complete band gaps hold the promise for future super-fast optical computing and optical communication technologies just as silicon semiconductors did for electronic computing and electronic communication technologies. To achieve a complete band gap, scientists have looked into a variety of different materials and structures. Besides silica spheres, different polymer spheres have also be used to produce opal-like structures (see, for example, Sanford A. Asher, et al. J. Am. Chem. Soc.
The present invention deals with a new type of physical, particles, colorants or pigments with opalescent effect, including 3-D structural color pigments. The particles of the present invention may contain a sphere based crystal structure (or super-lattice) built up by monodisperse spheres and one or more secondary types of much smaller colloidal particles occupying partially or completely the empty spaces between the monodisperse spheres. The smaller colloidal particles in the structure can modify the refractive index contrast between the silica spheres and the media in between them. They also may act as binding agents to hold the sphere based structure more strongly together. The monodisperse spheres suitable for the particles with opalescent effect typically have a standard deviation of the particle size of less than 5%, preferably about 2%.
The existing physical effect pigments or colorants are essentially exclusively based on layered structures with refractive indexes alternating in only one dimension. Typical products include, for example, pearlescent pigments based on bismuth oxychloride or lead carbonate crystalline platelets, interference pigments based mica flakes or alumina flakes, and gonio-chromatic pigments based on silica flakes, metal flakes or liquid crystal platelets. These products have been extensively reviewed by Pfaff and Reynders recently (Chemical Reviews, 1999, 99(7), 1963-1981).
This invention also deals with a method of manufacturing thin layer (flaky) crystals of monodisperse, desirably silica, spheres which can be useful as the 3-D structural color pigments using a substrate coating technology. The substrate may be a static, flat surface or a moving belt. The latter is known as a web coating process (U.S. Pat. No. 3,138,475) which was expanded to produce silica and titania flakes for layered interference colorants (World Patents 93/08237, 97/43346 and 97/43348). This web coating technology is capable of large scale production of 3-D structural color pigments of the current invention. The method may also be used to produce photonic crystals based on normal opal or inverse opal structures suitable for photonic and optoelectronic device applications.
Therefore, this invention may also deal with the use of the particles or of thin layer flaky crystals produced with the method described herein for photonic and optoelectronic device applications.